The present invention relates to a radio navigation apparatus and more particularly to an apparatus for automatically detecting a Loran-C signal.
Conventionally, navigation techniques called "Loran-A" and "Loran-C" which use the principle of hyperbolic navigation are used by ships and airplanes to pinpoint their positions. A Loran-C signal receiver determines the time lag between arrivals of signal pulses from master and secondary transmission stations. Toward this end, the receiver produces a sampling pulse signal, the period of which is equal to the pulse repetition period of the Loran-C signal, the first, second and third sampling pulses having the same phase as the pulses from the master, first and second secondary stations, respectively, and measures the time interval between the first and the second or third pulses. Therefore, it is necessary for the sampling pulse produced in the receiver in a random phase relationship with the master, first and second station pulses to be synchronized with the width of the pulses of the Loran signal. The Loran-C signal receiver receives a pulse-modulated 100-KHz sinuosoidal signal and measures the phases relationships among the carrier waves of the master and secondary station signals in order to measure the time differences therebetween.
An unexamined Japanese patent application TOKKAISHO No. 55-2938 of Haruo Kitamura published on Jan. 10, 1980 discloses an automatic pulse signal seeking apparatus.
As shown in FIG. 1 of the accompanying drawings, the apparatus detects a Loran-C pulse signal coming from master station. A Loran-C signal is received by an antenna 1, amplified by a high-frequency amplifier 2, gated by gates 3 and 4, held by hold units 5 and 6. A arithmetic operating unit 7 performs the arithmetic calculation z=.sqroot.x.sup.2 +y.sup.2 where x and y denote the outputs of hold units 5 and 6. A determiner 8 determines whether z is above a predetermined threshold value and outputs the result as a binary signal ("1" or "0"). A control unit 9 outputs a control signal depending on the output of determiner 8. An oscillator 10 generates a high-frequency clock pulse signal, the frequency of which is divided by a trigger generator 11 into a trigger pulse signal having a period equal to the period of the Loran-C signal pulses. The pulses of the trigger pulse signal are counted by a counter 12, the output of which controls a monostable multivibrator 14 which in turn provides a signal to a gate 15. A sampling pulse generator 13 produces sampling pulses in synchronism with the trigger pulse signal.
FIGS. 2(a) and 2(b) show the waveforms of signals produced by the major elements of the apparatus of FIG. 1. FIG. 2(b) is an enlargement of FIG. 2(a) where the waveform A shows the output of amplifier 2 and its envelope; the actual waveform consists of a 100-KHz carrier wave defining an envelope A by combination of its peak values. The waveforms B and C are sampling pulses inputted to gates 3 and 4, respectively, and in FIG. 2(a), the waveforms B and C are represented by a single common line segment. The waveform C is 2.5 .mu.s later than waveform B and corresponds to a 90-degree phase interval of the 100-KHz sinuosoidal wave. Thus, given the 100-KHz sinusoidal waveform K sin (2.pi.ft+.theta.) and the phase .theta..sub.1 of the waveform B, then the phase of the waveform C is (.theta..sub.1 +.pi./2), and the outputs of gates 3 and 4 are x=K sin .theta..sub.1 and y=K cos .theta..sub.1, respectively. Therefore, the output of calculating device 7 is z=.sqroot.x.sup.2 +y.sup.2 =K which is the peak value of the carrier. The Loran-C pulse signal consists of 8 or 9 pulses generated at predetermined time intervals. The waveform A of FIG. 2A is a pulse signal transmitted by the master station which consists of 8 pulses separated by 1 ms and a last pulse separated by 2 ms from the preceding pulse.
As described above, the output of arithmetic operating unit 7 represents the peak value K of the carrier of the Loran-C pulse signal. If the waveforms B and C have the timing relationship with waveform A shown in FIG. 2(b), K is zero so that determiner 8 outputs a zero signal which indicates that the input to the determiner is not higher than a predetermined threshold value. A non-zero output from determiner 8 turns on gate 15 via control unit 9.
After a predetermined number of trigger pulses are sent by trigger generator 11 to counter 12, i.e., after a predetermined number of measurements of the signal have been performed at the same phase so that the measured result can be deemed reliable, counter 12 produces a pulse which triggers monostable multivibrator 14 which in turn produces a pulse after a time lag T.sub.1 from the time of the triggering, thereby delaying the next generation of output pulses from trigger generator 11 by T.sub.1 via gate 15. As a result, the waveforms B and C approach the waveform A by T.sub.1. If this still leaves the input to determiner 8 below the predetermined threshold value, the waveforms B and C will be repeatedly delayed by T.sub.1 until the waveforms B and C match the timing of waveform A in FIGS. 2(a) and 2(b). In this case, the input to determiner 8 exceeds the threshold value, so that control unit 9 turns gate 15 off, thereby ending detection of the Loran signal timing.
In such automatic Loran-C detecting apparata, the time lag T.sub.1 by which the waveforms B and C are delayed must be shorter than the pulse width .tau. of envelope A. In addition, since .tau. is usually 200 .mu.s and the repetition period of Loran-C pulses is 99.7 ms in Japan, which is much longer than .tau., it takes a very long time to shift waveforms B and C by time lag T.sub.1 at a time until they match the timing of waveform A. For example, if T.sub.1 is 100 .mu.s, it could take as long as (99.7 ms/100 .mu.s).times.99.7 ms=9.9 seconds times the number of samples required for accuracy. Another problem is that in order for determiner 8 to detect the Loran signal, this signal must be greater than the predetermined threshold value and therefore a signal/noise (SN) ratio greater than a predetermined value must be ensured. When the Loran-C signal is received by vehicles, mountains, buildings, and overhead electrical cable for power transmission and communications could obstruct propagation and reception of the signals. In that case, the S/N ratio is often zero dB. Thus, this prior art apparatus is not suitable for a Loran-C signal detecting apparatus for a Loran-C signal receiver mounted on vehicles.